Cryptosporidium parvum (C. parum) is a coccidian protozoan that infects a wide variety of vertebrates, including humans. C. parvum can be acquired directly from animal-to-human contact, human-to-human contact or indirectly in fomites, water and sometimes food. Current et al. (1983) N. Engl. J. Med. 308:1252-1257; Centers of Disease Control (1984) “Cryptosporidiosis among children attending day-care centers—Georgia, Pennsylvania, Michigan, California, New Mexico” 33:599-601; Wolfson et al. (1995) N. Engl. J. Med. 312:1278-1281. Acquisition of infection occurs by ingestion of oocysts which excyst in the upper small bowel releasing four infective sporozoites. The sporozoites penetrate the lining enterocyte and undergo either sexual or asexual reproduction, gametogony and merogony, respectively. The products of either form of reproduction are capable of sustaining infection in man. Navin et al. (1984) Rev. Infect. Dis. 6:313-327.
The reservoir of C. parvum is in wild and domestic animals, particularly cattle (Tzipori (1983) Microbiol. Rev. 47:84-96) and this pathogen causes significant economic losses to the farm industry annually. Clinical manifestations of C. parvum infection may include watery diarrhea, crampy epigrastric abdominal pain, malabsorption of nutrients and weight loss, anorexia and malaise. The disease is usually self limited in the immunocompetent host but can be life threatening in the immunodeficient host, particularly in human patients with advanced Human Immunodeficiency Virus (HIV) infection. Current et al. (1983), supra; Wolfson et al. (1985) N. Engl. J. Med. 312:1278-1281; Soave (1988) Infect. Dis. Clin. N. Amer. 2:485.
The absence of an adequate in vitro culture system has severely limited the investigation of C. parvum. Furthermore, use of animal models, particularly mice, is of limited value because adult mice are not normally susceptible to infection with C. parvum. 
Several groups have reported the cloning and characterization of C. parvum antigens and genes using a variety of techniques. See, e.g., Jenkins et al. (1993) Infect. Immun. 61:2377-2382; Jenkins et al. (1995) Mol. Biochem. Parasitol. 71:149-152; Peterson et al. (1992) Infect. Immun. 60:2343-2348; Ranucci et al. (1993) Infect. Immun. 61:2347-2356.
After natural infection, a wide variety of cryptosporidial proteins are recognized by human and animal immune sera. Ortega-Mora et al. (1992) Infect. Immun. 60:3442-3445; Campbell et al. (1983) J. Clin. Microbiol. 18:165-169; Whitmire et al. (1991) Infect. Immun. 59:990-995; Nina et al. (1992) Infect. Immun. 60:1509-1513; Peeters et al. (1992) Infect. Immun. 60:2309-2316. The components that constitute protective immunity are unknown although, like most obligate intracellular coccidian protozoa, both the cellular and humoral arms of the immune response are likely to play important roles in the genesis of protective immunity. Lillehoj et al. (1994) Parasit. Today 10: 10-16. Use of recombinant C. parvum proteins in vaccine compositions has also been described. See, e.g., U.S. Pat. No. 5,591,434. However, no consistently effective therapy of vaccination exists, perhaps because the antigens used in the vaccines were identified from non-human sources.
There have also been several attempts to identify antibodies that neutralize C. parvum infection. In human studies, orally administered hyperimmune bovine colostrum has been found to alter the natural history of C. parvum by decreasing the excretion of oocysts, reducing the level of diarrhea, and in a smaller number of cases, clearing the parasite from stool. Tzipori et al. (1986) Br. Med. J. 293:1276-1277; Nord et al. (1989) “Treatment of AIDS associated cryptosporidiosis with hyperimmune colostrum from cows vaccinated with Cryptosporidium”, Fifth International Conference on AIDS, Montreal, Quebec, May 1989; Ungar et al. (1990) Gastroenterology 98:486-489. Several antigens that are recognized by sera after natural infection have been characterized and have been shown to be the targets of neutralizing antibody in the murine system. Arrowood et al. (1989) Infect. Immun. 57:2283-2288; Bjorneby et al. (1991) Infect. Immun. 59:1172-1176; Doyle et al. (1993) Infect. Immun. 61:4079-4084; Riggs et al. (1989) J. Immunol. 143:1340-1345; Peterson et al. (1992) Infect. Immun. 60:2343-2348. However, animal studies in the murine model of infection indicate only a partially protective role when these antibodies are orally administered.
Thus, there remains a need for the identification of antibodies useful in diagnostic and therapeutic compositions and for the identification of antigens from C. parvum that react with human sera and for diagnostic and therapeutic compositions and methods using these antigens.